Tenant Fit-Out: Detector Spacing, Battery, and Plenum Cable
You are the low-voltage contractor on a 14,000 sq ft medical office tenant fit-out in a four-story building. The base building has an addressable fire alarm control panel; your scope is 26 spot-type smoke detectors, 18 wall-mounted horn/strobes split across two notification appliance circuits, and the associated wiring. The tenant ceiling is a suspended acoustical grid at 9 ft with a 3 ft return-air plenum above it; the longest corridor is 62 ft long and 8 ft wide. The plans show FPLR cable in the plenum, a single smoke detector in the corridor, and reuse of the existing 7 Ah sealed battery. Panel records show a standby load of 0.35 A and an alarm load of 4.8 A. The building official has asked for the fire alarm shop drawings with detector spacing and battery calculations before issuing the permit, and the electrician has already installed a 1 in. EMT sleeve that your cable must share with a 120 V lighting circuit for 40 ft.
1. Lay out smoke detection for the 62 ft corridor and explain the rule you applied.
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Rule set
NFPA 72 spot-type smoke detector spacing on a smooth ceiling is 30 ft between detectors, with the last detector no more than 21 ft (0.7 x 30) from a sidewall or end wall. A corridor less than 15 ft wide may be covered by a single row along the centerline. Keep detectors at least 4 in. from walls and at least 3 ft from supply diffusers.
Layout
62 ft long: a single detector at midspan sits 31 ft from each end wall, which exceeds 21 ft - the plan on the drawings is deficient. Use two detectors at 20 ft and 42 ft from one end wall:
- 20 ft to the near end wall (OK, under 21 ft)
- 20 ft to the far end wall (OK)
- 22 ft between detectors (OK, under 30 ft)
Notes for the shop drawings
- Show the corridor as a single row on the centerline.
- Verify the 9 ft ceiling is smooth and unobstructed; beams deeper than 18 in. or ceiling heights above 10 ft change the spacing from the NFPA 72 tables.
- Coordinate with the diffuser layout before the ceiling grid is set; relocating a detector after grid installation is a change order.
2. Check the battery. Is the existing 7 Ah unit adequate for this tenant space?
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Capacity method
Commercial systems are sized for 24 hours of standby followed by 5 minutes of alarm (NFPA 72, secondary power supply).
- Standby: 0.35 A x 24 h = 8.4 Ah
- Alarm: 4.8 A x (5/60) h = 0.40 Ah
- Subtotal = 8.8 Ah
- Apply a 20% aging/derating allowance: 8.8 x 1.2 = 10.56 Ah required
A 7 Ah battery is roughly 34% short. Select a 12 Ah battery at minimum (18 Ah if the calculated load will grow with future tenant devices).
Two follow-on checks
- Charger: the panel's charging circuit must be rated to recharge the larger battery within the code time frame; a 7 Ah charger setting on a 12 Ah battery will leave the system chronically undercharged.
- Alarm duration: confirm whether the system is a voice evacuation system or has a longer alarm duration requirement in the edition of NFPA 72 adopted in Massachusetts - 5 minutes is the baseline, not a universal answer.
- Document both calculations on the shop drawings; the building official asked for them.
3. The plans call for FPLR in the plenum and the cable must share a 1 in. EMT sleeve with a 120 V lighting circuit for 40 ft. What do you direct, and why?
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Cable rating in the plenum
Above a suspended ceiling used as a return-air plenum, 300.22(C) requires metal raceway, metal-clad cable, or plenum-listed cable. FPLR is a riser rating and is not acceptable in the plenum; the exposed runs must be FPLP (or run in metal raceway for their entire length in the plenum). Substitution runs one way only: a plenum-rated cable may be used where a riser rating is required, never the reverse.
Sharing the sleeve
Power-limited fire alarm conductors may not share a raceway with power conductors unless separated by a barrier or covered by a specific Article 760 exception. Sharing the 1 in. EMT with the lighting circuit is a violation as drawn, and the EMT will also be crowded.
Direction to the crew
- Submit an RFI replacing FPLR with FPLP throughout the plenum.
- Give the fire alarm its own raceway; if the sleeve must be shared, install a listed divider and confirm the fill, but a separate sleeve is cleaner and easier to inspect.
- Keep FPLP jacket markings visible at each accessible point and photograph the separation before the ceiling closes.